In automobiles in recent years, in order to relieve an impact load received by a driver, which results from secondary collision between the driver and a steering wheel upon an accident, a steering apparatus in which secondary collision energy is absorbed by contraction of a steering column formed of two components: an inner column and an outer column has been widely adopted. In this kind of steering apparatus, the steering wheel moves toward a front side with respect to a vehicle by relative sliding of the inner column and the outer column in an axial direction, and the secondary collision energy is absorbed by an energy absorption means provided between the inner column and the outer column.
For example, in a steering apparatus described in PTL 1, an outer column arranged on a lower side is attached to a vehicle body through a tilt bracket and a tilt pivot, and an inner column arranged on an upper side is held on the outer column by being fastened with a tilt/telescopic adjustment mechanism. In this steering apparatus, an amount in which the inner column can enter into the outer column upon the secondary collision is set to be larger than the amount upon telescopic adjustment, and the inner column moves toward the front side with respect to the vehicle against friction force generated by fastening with the tilt/telescopic adjustment mechanism.
In such a steering apparatus, it is required to smoothly absorb the secondary collision energy when a light-weight driver collides with the steering wheel. In order to achieve such a requirement, it is considered to reduce the fastening force with the tilt/telescopic adjustment mechanism. However, when the fastening force is reduced, a force holding the inner column is reduced, so that backlash is easily caused in a fitting portion between the inner column and the outer column. Therefore, in PTL 1, low-friction material treatment by coating or the like is applied to an outer peripheral surface of the inner column, or an inner peripheral surface of the outer column to reduce the friction force generated by fastening without reducing the fastening force.
Moreover, in a telescopic type steering apparatus, a steering shaft is generally constructed by an inner shaft and an outer shaft spline-connected to each other in the steering column in order to satisfy both transmission of steering torque and telescopic adjustment. In this case, in order to prevent backlash noise caused by a fine clearance between a male spline and a female spline, resin coating is applied to one of both of the splines in several cases.